#!/usr/bin/env python
#-*- coding:utf-8 -*-
import types
def score(point):   
    if type(point) is types.StringType: return None
    level = 'A'
    if( point < 60 ):
        level = 'F'
    elif (point >= 60 and point <= 69):
        level = 'D'
    elif (point >= 70 and point <= 79):
        level = 'C'
    elif (point >= 80 and point <= 89):
        level = 'B'
    else:
        level = 'A'
    return level

if __name__ == '__main__':
    print score('90')
